In a standard deep-drilling operation a drill string formed by a plurality of longitudinally interconnected drill rods, e.g. about 10 m long, or a single so-called tubing/pipe carries at its lower end a drill bit and is suspended from its upper end by a pulley block of a derrick. The bit is rotated either by its own motor in the lower end of the drill string or by rotating the entire string as it is lowered from the derrick so that the bit works its way downward. Drilling mud of high specific gravity is pumped from above ground down through the tubular string to emerge at its lower end and then move back upward in the hole around the string, cooling the bit and bringing to the surface the debris cut by the bit.
As the string moves downward, new drill rods are added to its upper end. The connection is made by forming the one end, normally the upper end, of each section with a shallow internally threaded, downwardly tapered, and frustoconical recess and the opposite end with a short downwardly tapered and externally threaded stub. When threaded together the two frustoconical formations lock extremely tightly, often with some plastic deformation to form an extremely solid joint not only capable of transmitting considerable torque, but also forming a very tight leakproof joint.
With deep-drilling operations it is possible to reach depths of 5000 m to 10,000 m. The drilling mud creates in the hole a pressure that increases downward, increasing by 1 bar to 2 bar for every 10 m. This hydrostatic pressure is effective axially, that is vertically and longitudinally of the drill string, on the drill head carrying the drill bit, while at the upper end only atmospheric pressure is bearing on the drill string and in fact the derrick is normally applying some upward tension on the string.
In the context of known measures (Encyclopedia of Natural Science and Technology, Modern Industry; Munich 1980; pages 1233-1234) the drill string comprises one unit with respect to the hydrostatic effects which the drilling mud exerts on it. The hydrostatic pressure of the drilling mud acts on the rotary drill bit within the framework of a state of hydrostatic stress, with an axial component with respect to the drill string which at its upper end merely acts in opposition to atmospheric pressure. The radial components of the upwardly decreasing hydrostatic pressure of the drilling mud, which are distributed over the length of the drills string, cancel each other out. In this way the hydrostatic pressure of the drilling mud acts away from the rotary drilling bit with a component of pressure acting on the bottom end of the drill string. This exerts a lifting force on the drills string which opposes the weight of the drill string acting in the axial direction. The lifting force and weight are superimposed and lead to elastic deformations which result from the state of stress.
A so-called neutral zone is located between a lower section of the drill string with a compressive stress which decreases upward and an upper section of the drill string with a tensile stress which decreases downward. It is understood that the conditions and the position of the neutral zone are altered if the drill string rests on the base of the bore hole with a supporting force which is more or less greater according to the conditions imposed on the hook from which the drill string hangs. It is further understood that for a constant supporting force the neutral zone moves upward within the drill string corresponding to the progress of the drilling and thus corresponding to the increasing hydrostatic pressure with respect to the base of the bore hole.
If a drill string or a drill-string section is placed under a compressive stress, its progress in a straight line is a metastable state. As soon as it is acted upon by lateral forces the drill string can buckle. Since the diameter of a bore hole is, as a rule, larger than the outside diameter of the drill pipes, that part of the drill string which extends from the base of the bore hole to the neutral zone is in danger of buckling when drilling takes place. The direction of drilling of the rotary drill bit can depart from the vertical if buckling takes place. Undesirable deviations in the bore hole can then occur. Moreover due to buckling there is an increased danger of the drill rods breaking in the region of compressive stress.
Within the framework of the known measures, The lower end of the drill string is formed with weights which are constructed as drill rods of heavy wall thickness and therefore great mass. There are also heavy rods which are formed in such a way and arranged in such a number that the drill string in the region between the rotary drilling bit and the neutral zone is stronger and therefore has a reduced tendency to buckle under the influence of the above-mentioned hydrostatic compressive stresses and, if applicable, the compressive stresses determined by the supporting force. For a deep bore hole the part of the drill string which consists of heavy rods can be several hundred meters long.
With the known measures for sinking a deep bore hole, deviations are not eliminated but are merely reduced. Moreover, the bore-hole deviations increase with increasing depth of the bore hole. The use of an increasing number of heavy rods as drilling progresses, due to the increasing hydrostatic pressure, leads to an increase in the load on the hook as the depth of the bore hole increases. The cost of manufacturing and assembling a large number of special heavy rods is high.
It is further standard (see Applied Drilling Engineering of A. Bourgoyne (Society of Petroleum Engineers; 1986; p. 122f) to provide a weight collar at the lower end of the bit to move down this neutral zone, and in addition to provide a coupling or joint immediately above the drill bit (see U.S. Pat. No. 4,281,726 of Garrett). Such a known coupling is comprised of two relatively sealed hollow or tubular parts that can move relative to each other axially of the drill string. Their relative axial movability is considerable and each of the parts is fixed to a respective section of the drill string. The two parts may be provided with interengaging formations--splines for instance--which rotationally couple them together, although in a bottom-drive system such coupling may not be necessary. During operation the coupling is under compression and there is no hydro-static decoupling.
In the parent applications I describe an improvement on a drilling method wherein a vertical drill string formed by a plurality of longitudinally joined hollow drill rods has a lower end provided with a bit and is simultaneously rotated and lowered into the ground to form therein a bore of a predetermined diameter and drill mud is pumped through the string to emerge at the lower end thereof and then flow upward in the bore around the string. According to this prior invention the lower end of the string is weighted at the bit and an upward force is applied to an upper end of the string to place the entire string between its ends under tension. The string is provided between its ends with at least one outwardly projecting compensator joint that forms in the bore a constriction in turn forming pressurizable regions such that pressure from the drilling mud is upwardly and downwardly effective in the regions on the coupling.
Thus according to my prior invention hydrostatically independent sections of the drill string are formed in each of which the axial force is formed by the difference between the hydrostatic pressure at the top and at the bottom of the sections.
It follows from the definition of the hydrostatically independent drill-string section that the hydrostatically determined axial stress merely results from the difference in hydro-static pressure between the upper and lower ends of the hydro-statically independent drill-string sections. Thus this stress is proportional to the length of the drill-string section. The latter is selected so that a hydrostatically induced risk of buckling does not arise in the drill-string section, even under the possible effect of the supporting force. A hydrostatically independent drill-string section which is hung in the drilling mud and which is acted upon at its upper end and at its lower end by the hydrostatic pressure of the drilling mud in the bore hole has a compressive stress zone, a neutral zone without compressive stress or tensile stress, and a tensile-stress zone. The position of this neutral zone is thus substantially independent of the depth to which the drill-string section is submerged since it is only the difference in hydrostatic pressure between the upper and lower ends which determines the length of the compressive-stress zone. If a second drill-string section is hung from such a submerged drill-string section, the first drill-string section is placed completely under a tensile stress since the second section exerts a tensile force which exceeds the compressive force on the first section resulting from the hydrostatic pressure difference (a prerequisite for this is that the sections have the same dimensions). The same applies if further sections are hung on. A drill string of this type behaves physically like a chain which is hung in a liquid. A zone of compressive stress only exists in the lower region of the lowest section. Each link of the chain forms a separate unit with substantially balanced upward and downward forces.
If a drill string comprising hydrostatically independent sections is fitted at its lower end with a rotating drill bit and is lowered far enough that the rotary drill bit is located at the base of the bore hole, a compressive load is exerted on the rotary drill bit not by the drill string itself but by a weight of constant mass which is supported near the rotary drill bit. On drilling, a compressive load acts on the rotary drill bit which is independent of the hydrostatic pressure at the base of the bore hole. The compressive load on the rotary drill bit is determined by the mass of the weight minus the hydrostatic pressure difference at the lowest drill-string section. In contrast, the hydrostatic pressure difference for known processes is determined by the complete drill string. In order to produce a definite compressive load on the rotary drill bit, only one (or a few) weight is required for the process, the weight of which is less than the total weight of the heavy rods in the known processes. Furthermore the load on the hook is reduced.
In the drilling system due to the support to the weight in the immediate vicinity of the rotary drill bit, the entire drill string located above the support is under a tensile stress and the drill string cannot buckle. As a consequence, bore-hole deviations are substantially prevented and drilling progresses along a straight line in the vertical direction.
According to details of my prior invention there are a plurality of hydrostatic compensator joints in the string forming in the bore constrictions in turn forming pressurizable regions such that pressure from the drilling mud is upwardly and downwardly effective in the regions on the couplings. Each hydrostatic compensator joint comprises an upper part fixed to one of the rods, a lower part fixed to another of the rods immediately beneath the upper part, formations rotationally coupling the parts together, and limiting formations permitting only very limited relative axial movement between the upper and lower parts. The two parts are coaxial axially interfitting sleeves and the hydrostatic compensator joints each further have a seal between the parts preventing leakage from between them. The coupling formations are axially interengaging teeth and the limiting formations only permitting about 5 mm of relative axial movement. Furthermore but not necessarily, a spring is provided that is braced axially between the parts to urge them axially apart.
The weight in accordance with this earlier invention is formed as a tube coaxial with the string and having an inner diameter that is greater than an outside diameter of the string and a lower end that is attached to the string at the bit. The weight is axially decoupled from the string upward of its lower end. The weight can also have an inner diameter that is smaller than an outside diameter of the string.
The parent applications also disclose a system wherein a collar weight is fitted at the lower end of the string at the bit and at least one hydrostatic compensator joint is provided in the string, preferably immediately above the weight. The hydrostatic compensator joint has a tubular upper part connected to the string above the hydrostatic compensator joint, a tubular lower part connected to the string below the hydrostatic compensator joint and longitudinally limitedly shiftable in the upper part, and means including a seal between the parts.
While these systems offer substantial advantages, they still do not fully recreate in the drill string the desired stress-free relationships seen, for instance, in a chain hung in a body of liquid.